There is an electric power steering apparatus (EPS) as an apparatus provided a motor control apparatus, and the electric power steering apparatus assist-controls a steering mechanism of a vehicle by using a rotational force (assist force) of a motor. That is, a driving force of the motor controlled by a power supplied from a power supplying section (inverter) is applied to a steering shaft or a rack shaft by means of a transmission mechanism such as gears. In order to accurately generate the steering assist torque, such a conventional electric power steering apparatus performs a feedback control of a motor current. The feedback control adjusts a voltage supplied to the motor so that a difference between a steering assist command value (a current command value) and a detected motor current value becomes small, and the adjustment of the voltage applied to the motor is generally performed by an adjustment of duty command values of a pulse width modulation (PWM) control.
A general configuration of a conventional electric power steering apparatus will be described with reference to FIG. 1. As shown in FIG. 1, a column shaft (a steering shaft, a handle shaft) 2 connected to a handle 1, is connected to steered wheels 8L and 8R through reduction gears 3, universal joints 4a and 4b, a rack and pinion mechanism 5, and tie rods 6a and 6b, further via hub units 7a and 7b. Further, the column shaft 2 is provided with a torque sensor 10 for detecting a steering torque Th of the handle 1 and a steering angle sensor 14 to detect a steering angle θ, and a motor 20 for assisting the steering force of the handle 1 is connected to the column shaft 2 through the reduction gears 3. Electric power is supplied to a control unit (an ECU) 100 for controlling the electric power steering apparatus from a battery 13, and an ignition key signal is inputted into the control unit 100 through an ignition key 11. The control unit 100 calculates a current command value of an assist (steering assist) command based on the steering torque Th detected by the torque sensor 10 and a vehicle speed Vel detected by a vehicle speed sensor 12, and controls a current supplied to the motor 20 for EPS based on a voltage control command value Vref obtained by performing compensation and so on with respect to the current command value in a current control section.
The steering angle sensor 14 is not always necessary and it is possible to remove. Further, it is possible to get the steering angle from a rotational sensor such as a resolver connected to the motor 20.
A controller area network (CAN) 50 to send/receive various information and signals on the vehicle is connected to the control unit 100, and it is also possible to receive the vehicle speed Vel from the CAN 50. Further, a Non-CAN 51 is also possible to connect to the control unit 30, and the Non-CAN 51 sends and receives a communication, analogue/digital signals, electric wave or the like except for the CAN 50.
The control unit 100 mainly comprises a CPU (or an MPU or an MCU), and general functions performed by programs within the CPU are shown in FIG. 2.
Functions and operations of the control unit 100 will be described with reference to FIG. 2. As shown in FIG. 2, the steering torque Th detected by the torque sensor 10 and the vehicle speed Vel detected (or from the CAN 50) by the vehicle speed sensor 12 are inputted into a current command value calculating section 101. The current command value calculating section 101 decides a current command value Iref1 that is a desired value of the current supplied to the motor 20 based on the steering torque Th and the vehicle speed Vel and by means of an assist map and so on. The current command value Iref1 is inputted into a current limiting section 103 through an adding section 102A. A current command value Irefm that is limited the maximum current, is inputted into a subtracting section 102B for the feedback, and a deviation I (=Irefm−Im) between the current command value Irefm and a motor current value Im that is fed back, is calculated. The deviation I is inputted into a PI-control section 104 to improve a characteristic of the steering operation. The voltage command value Vref that characteristic improvement is performed in the PI-control section 104, is inputted into a PWM-control section 105. Furthermore, the motor 20 is PWM-driven through an inverter 106 serving as a drive section. The current value Im of the motor 20 is detected by a current detector 107 and is fed back to the subtracting section 102B. The inverter 106 uses EFTs (field-effect transistors) as switching elements and is comprised of a bridge circuit of FETs.
A rotational sensor 21 such as a resolver is connected to the motor 20, a motor rotational angle θ is outputted from the rotational sensor 21, and further a motor velocity ω is calculated in a motor velocity calculating section 22
Further, a compensation signal CM from a compensating section 110 is added in the adding section 102A, and the compensation of the system is performed by the addition of the compensation signal CM so as to improve a convergence, an inertia characteristic and so on. The compensating section 110 adds a self-aligning torque (SAT) 113 and an inertia 112 in an adding section 114, further adds the result of addition performed in the adding section 114 and a convergence 111 in an adding section 115, and then outputs the result of addition performed in the adding section 115 as the compensation signal CM.
In the case that the motor 20 is a 3-phase (U-phase, V-phase and W-phase) brushless motor, details of the PWM-control section 105 and the inverter 106 are a configuration such as shown in FIG. 3. The PWM-control section 105 comprises a duty calculating section 105A that calculates PWM-duty command values D1˜D6 of three phases according to a given expression based on the voltage control command value Vref and a gate driving section 105B that drives gates of FETs as serving drive elements with the PWM-duty command values D1˜D6 and ON/OFF-switches after the compensation of the dead time. Further, the inverter 106 comprises a three-phase bridge (FET1˜FET6) of FETs as serving semiconductor switching elements and drives the motor 20 by being ON/OFF-switched based on the PWM duty command values D1˜D6. A motor relay 23 to supply (ON) a power and block (OFF) is provided at power supplying lines between the inverter 106 and the motor 20.
In such the above electric power steering apparatus, there are cases to encounter an unexpected situation of a system abnormality detection time (for example, a disconnection of the torque sensor, a short-circuit accident of the motor control stage-FETs and so on). AS a counterplan for these cases, first, an assist control of the electric power steering apparatus is instantly stopped and a connection a drive control system and the motor is cut-offed.
Generally, as shown in FIG. 3, the motor relay 23 for supplying/blocking the motor current is provided between the motor 20 and the inverter 106 to control the current flowing in the motor 20. A cheap contact relay is used for the motor relay 23, and the current flowing the motor 20 is cut-offed in a hard-wear by electromagnetically releasing the contact (e.g. Japanese Published Unexamined Patent Application No. 2005-199746 A (Patent Document 1)).
However, recently, a motor release-switch comprising non-contact semiconductor switching elements (analogue switches), for example FETs, has been used in place of the contact electromagnetic motor relay for the improvement of a miniaturization and a reliability as well as a cost reduction. But, when the assist continuation becomes impossible due to the system abnormality, the motor is rotated even if the inverter is stopped. At this time, if the motor release-switch is OFF-switched during the motor rotation, a regenerative current of the motor deviates an area of safety operation of the motor release-switch and there is a case that the motor release-switch is broken or destroyed.